Timing refractory period for cardiac resynchronization pacing

ABSTRACT

A device and method for cardiac rhythm management in which a heart chamber is paced in accordance with a pacing mode that employs sense signals from the opposite chamber. A sensing refractory period is provided in order to prevent the pacing interval from being lengthened due to delays in conduction of excitation from the paced chamber to the sensed chamber.

CROSS-REFERENCE TO RELATED APPLICATION(S)

[0001] This application is a continuation of U.S. patent applicationSer. No. 09/852,125, filed on May 9, 2001, the specification of which isincorporated herein by reference.

FIELD OF THE INVENTION

[0002] This invention pertains to methods and apparatus for cardiacrhythm management. In particular, the invention relates to methods andapparatus for providing cardiac resynchronization pacing.

BACKGROUND

[0003] Cardiac rhythm management devices are implantable devices thatprovide electrical stimulation to selected chambers of the heart inorder to treat disorders of cardiac rhythm and include pacemakers andimplantable cardioverter/defibrillators. A pacemaker is a cardiac rhythmmanagement device that paces the heart with timed pacing pulses. Themost common condition for which pacemakers are used is in the treatmentof bradycardia, where the ventricular rate is too slow.Atrio-ventricular conduction defects (i.e., AV block) that are permanentor intermittent and sick sinus syndrome represent the most common causesof bradycardia for which permanent pacing may be indicated. Iffunctioning properly, the pacemaker makes up for the heart's inabilityto pace itself at an appropriate rhythm in order to meet metabolicdemand by enforcing a minimum heart rate. Pacing therapy may also beapplied in order to treat cardiac rhythms that are too fast, termedanti-tachycardia pacing. (As the term is used herein, a pacemaker is anycardiac rhythm management device with a pacing functionality, regardlessof any other functions it may perform such as the delivery cardioversionor defibrillation shocks to terminate atrial or ventricularfibrillation.)

[0004] Also included within the concept of cardiac rhythm is the mannerand degree to which the heart chambers contract during a cardiac cycleto result in the efficient pumping of blood. For example, the heartpumps more effectively when the chambers contract in a coordinatedmanner. The heart has specialized conduction pathways in both the atriaand the ventricles that enable the rapid conduction of excitation (i.e.,depolarization) throughout the myocardium. These pathways conductexcitatory impulses from the sino-atrial node to the atrial myocardium,to the atrio-ventricular node, and thence to the ventricular myocardiumto result in a coordinated contraction of both atria and bothventricles. This both synchronizes the contractions of the muscle fibersof each chamber and synchronizes the contraction of each atrium orventricle with the contralateral atrium or ventricle. Without thesynchronization afforded by the normally functioning specializedconduction pathways, the heart's pumping efficiency is greatlydiminished. Patients who exhibit pathology of these conduction pathways,such as bundle branch blocks, can thus suffer compromised cardiacoutput.

[0005] Patients with conventional pacemakers can also have compromisedcardiac output because artificial pacing with an electrode fixed into anarea of the myocardium does not take advantage of the above-describedspecialized conduction system. The spread of excitation from a singlepacing site must proceed only via the much slower conducting musclefibers of either the atria or the ventricles, resulting in the part ofthe myocardium stimulated by the pacing electrode contracting wellbefore parts of the chamber located more distally to the electrode,including the myocardium of the chamber contralateral to the pacingsite. Although the pumping efficiency of the heart is somewhat reducedfrom the optimum, most patients can still maintain more than adequatecardiac output with artificial pacing.

[0006] Heart failure is a clinical syndrome in which an abnormality ofcardiac function causes cardiac output to fall below a level adequate tomeet the metabolic demand of peripheral tissues and is usually referredto as congestive heart failure (CHF) due to the accompanying venous andpulmonary congestion. CHF can be due to a variety of etiologies withischemic heart disease being the most common. Some CHF patients sufferfrom some degree of AV block or are chronotropically deficient such thattheir cardiac output can be improved with conventional bradycardiapacing. Such pacing, however, may result in some degree ofuncoordination in atrial and/or ventricular contractions due to the wayin which pacing excitation is spread throughout the myocardium asdescribed above. The resulting diminishment in cardiac output may besignificant in a CHF patient whose cardiac output is alreadycompromised. Intraventricular and/or interventricular conduction defectsare also commonly found in CHF patients. In order to treat theseproblems, cardiac rhythm management devices have been developed whichprovide electrical pacing stimulation to one or more heart chambers inan attempt to improve the coordination of atrial and/or ventricularcontractions, termed cardiac resynchronization therapy.

SUMMARY OF THE INVENTION

[0007] Cardiac resynchronization therapy can most conveniently bedelivered by a cardiac rhythm management device in accordance with abradycardia pacing mode so that the activation patterns between andwithin selected heart chambers are both resynchronized and pacedconcurrently. One way to implement resynchronization therapy is todesignate one heart chamber as the rate chamber and the contralateralchamber as the synchronized chamber and then pace one or both chambersbased upon rate chamber senses. In one particular resynchronizationpacing mode, only the synchronized chamber is paced in accordance with ademand pacing algorithm defined with respect to rate chamber senses. Forexample, if the right and left ventricles are designated as the rate andsynchronization chambers, only the left ventricle is paced in accordancewith a conventional bradycardia pacing mode defined with respect toright ventricular sense signals. That is, a left ventricular pace isdelivered at a pacing instant that occurs upon expiration of an escapeinterval without receiving a right ventricular sense, where the escapeinterval is reset upon a right ventricular sense or after delivery of aleft ventricular pace.

[0008] Pacing only the synchronized chamber in this manner means thatthe rate chamber will be depolarized subsequent to the delivery of thepace as excitation is conducted from the synchronized chamber to therate chamber. Although the rate chamber sensing channel may be renderedrefractory upon delivery of the pace, if the conduction time is longerthan the refractory period, a rate chamber sense of the depolarizationresulting from the pace will reset the escape interval. This willnecessarily decrease the rate at which paces are delivered below thatwhich is desired by lengthening the pacing interval. The presentinvention therefore employs a selected timing refractory period,initiated by a pace to the synchronized chamber, during which ratechamber senses are ignored for purposes of resetting the escapeinterval. Rate chamber senses occurring within the timing refractoryperiod due to conduction of excitation resulting from a pace then do notaffect the pacing rate but may be counted for purposes of detecting atachyarrhythmia in the rate chamber.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]FIG. 1 is a system diagram of a pacemaker configured forresynchronization pacing.

[0010] FIGS. 2A-B are timing diagrams showing right ventricular sensesas a result of a conducted left ventricular pace.

[0011] FIGS. 3A-B are timing diagrams illustrating the timing refractoryperiod.

DETAILED DESCRIPTION

[0012] The present invention is concerned with a method and system fordelivering cardiac resynchronization pacing in a particular mode. Thefollowing is a description of the hardware used to deliver such therapy,of resynchronization pacing in general, and an exemplary embodiment ofthe invention.

[0013] 1. Hardware Platform

[0014] Pacemakers are typically implanted subcutaneously on a patient'schest and have leads threaded intravenously into the heart to connectthe device to electrodes used for sensing and pacing. A programmableelectronic controller causes the pacing pulses to be output in responseto lapsed time intervals and sensed electrical activity (i.e., intrinsicheart beats not as a result of a pacing pulse). Pacemakers senseintrinsic cardiac electrical activity by means of internal electrodesdisposed near the chamber to be sensed. A depolarization wave associatedwith an intrinsic contraction of the atria or ventricles that isdetected by the pacemaker is referred to as an atrial sense orventricular sense, respectively. In order to cause such a contraction inthe absence of an intrinsic beat, a pacing pulse (either an atrial paceor a ventricular pace) with energy above a certain pacing threshold isdelivered to the chamber.

[0015]FIG. 1 shows a system diagram of a microprocessor-based pacemakerphysically configured with sensing and pacing channels for both atriaand both ventricles. The controller 10 of the pacemaker is amicroprocessor that communicates with a memory 12 via a bidirectionaldata bus. The memory 12 typically comprises a ROM (read-only memory) forprogram storage and a RAM (random-access memory) for data storage. Thepacemaker has atrial sensing and pacing channels comprising electrode 34a-b, leads 33 a-b, sensing amplifiers 31 a-b, pulse generators 32 a-b,and atrial channel interfaces 30 a-b which communicate bidirectionallywith microprocessor 10. The device also has ventricular sensing andpacing channels for both ventricles comprising electrodes 24 a-b, leads23 a-b, sensing amplifiers 21 a-b, pulse generators 22 a-b, andventricular channel interfaces 20 a-b. In the figure, “a” designates oneventricular or atrial channel and “b” designates the channel for thecontralateral chamber. In this embodiment, a single electrode is usedfor sensing and pacing in each channel, known as a unipolar lead. Otherembodiments may employ bipolar leads that include two electrodes foroutputting a pacing pulse and/or sensing intrinsic activity. The channelinterfaces 20 a-b and 30 a-b include analog-to-digital converters fordigitizing sensing signal inputs from the sensing amplifiers andregisters which can be written to by the microprocessor in order tooutput pacing pulses, change the pacing pulse amplitude, and adjust thegain and threshold values for the sensing amplifiers. An exertion levelsensor 330 (e.g., an accelerometer or a minute ventilation sensor)enables the controller to adapt the pacing rate in accordance withchanges in the patient's physical activity. A telemetry interface 40 isalso provided for communicating with an external programmer 500 whichhas an associated display 510. A pacemaker incorporating the presentinvention may possess all of the components in FIG. 1 and beprogrammable so as to operate in a number of different modes, or it mayhave only those components necessary to operate in a particular mode.

[0016] The controller 10 controls the overall operation of the device inaccordance with programmed instructions stored in memory. The controller10 controls the delivery of paces via the pacing channels, interpretssense signals from the sensing channels, and implements timers fordefining escape intervals and sensory refractory periods. Bothbradycardia and anti-tachycardia pacing modes may be implemented in codeexecuted by the controller. In the latter, paces are delivered accordingto a defined protocol that acts so as to terminate a tachyarrhythmiawhen such a rhythm is detected by the sensing channels.

[0017] 2. Bradycardia Pacing Modes

[0018] Bradycardia pacing modes refer to pacing algorithms used to pacethe atria and/or ventricles when the intrinsic ventricular rate isinadequate either due to AV conduction blocks or sinus node dysfunction.Such modes may either be single-chamber pacing, where either an atriumor a ventricle is paced, or dual-chamber pacing in which both an atriumand a ventricle are paced. The modes are generally designated by aletter code of three positions where each letter in the code refers to aspecific function of the pacemaker. The first letter refers to whichheart chambers are paced and which may be an A (for atrium), a V (forventricle), D (for both chambers), or O (for none). The second letterrefers to which chambers are sensed by the pacemaker's sensing channelsand uses the same letter designations as used for pacing. The thirdletter refers to the pacemaker's response to a sensed P wave from theatrium or an R wave from the ventricle and may be an I (for inhibited),T (for triggered), D (for dual in which both triggering and inhibitionare used), and O (for no response). Modem pacemakers are typicallyprogrammable so that they can operate in any mode which the physicalconfiguration of the device will allow. Additional sensing ofphysiological data allows some pacemakers to change the rate at whichthey pace the heart in accordance with some parameter correlated tometabolic demand. Such pacemakers are called rate-adaptive pacemakersand are designated by a fourth letter added to the three-letter code, R.

[0019] Pacemakers can enforce a minimum heart rate either asynchronouslyor synchronously. In asynchronous pacing, the heart is paced at a fixedrate irrespective of intrinsic cardiac activity. There is thus a riskwith asynchronous pacing that a pacing pulse will be deliveredcoincident with an intrinsic beat and during the heart's vulnerableperiod which may cause fibrillation. Most pacemakers for treatingbradycardia today are therefore programmed to operate synchronously in aso-called demand mode where sensed cardiac events occurring within adefined interval either trigger or inhibit a pacing pulse. Inhibiteddemand pacing modes utilize escape intervals to control pacing inaccordance with sensed intrinsic activity. In an inhibited demand mode,a pacing pulse is delivered to a heart chamber during a cardiac cycleonly after expiration of a defined escape interval during which nointrinsic beat by the chamber is detected. If an intrinsic beat occursduring this interval, the heart is thus allowed to “escape” from pacingby the pacemaker. Such an escape interval can be defined for each pacedchamber. For example, a ventricular escape interval can be definedbetween ventricular events so as to be restarted with each ventricularsense or pace. The inverse of this escape interval is the minimum rateat which the pacemaker will allow the ventricles to beat, sometimesreferred to as the lower rate limit (LRL).

[0020] In atrial tracking pacemakers (i.e., VDD or DDD mode), anotherventricular escape interval is defined between atrial and ventricularevents, referred to as the atrio-ventricular interval (AVI). Theatrio-ventricular interval is triggered by an atrial sense or pace andstopped by a ventricular sense or pace. A ventricular pace is deliveredupon expiration of the atrio-ventricular interval if no ventricularsense occurs before. Atrial-tracking ventricular pacing attempts tomaintain the atrio-ventricular synchrony occurring with physiologicalbeats whereby atrial contractions augment diastolic filling of theventricles. If a patient has a physiologically normal atrial rhythm,atrial-tracking pacing also allows the ventricular pacing rate to beresponsive to the metabolic needs of the body.

[0021] A pacemaker can also be configured to pace the atria on aninhibited demand basis. An atrial escape interval is then defined as themaximum time interval in which an atrial sense must be detected after aventricular sense or pace before an atrial pace will be delivered. Whenatrial inhibited demand pacing is combined with atrial-triggeredventricular demand pacing (i.e., DDD mode), the lower rate interval isthen the sum of the atrial escape interval and the atrio-ventricularinterval.

[0022] Finally, rate-adaptive algorithms may be used in conjunction withbradycardia pacing modes. Rate-adaptive pacemakers modulate theventricular and/or atrial escape intervals based upon measurementscorresponding to physical activity. Such pacemakers are applicable tosituations in which atrial tracking modes cannot be use. In arate-adaptive pacemaker operating in a ventricular pacing mode, the LRLis adjusted in accordance with exertion level measurements such as froman accelerometer or minute ventilation sensor in order for the heartrate to more nearly match metabolic demand. The adjusted LRL is thentermed the sensor-indicated rate.

[0023] 3. Cardiac Resynchronization Therapy

[0024] Cardiac resynchronization therapy is pacing stimulation appliedto one or more heart chambers in a manner that restores or maintainssynchronized bilateral contractions of the atria and/or ventricles andthereby improves pumping efficiency. Certain patients with conductionabnormalities may experience improved cardiac synchronization withconventional single-chamber or dual-chamber pacing as described above.For example, a patient with left bundle branch block may have a morecoordinated contraction of the ventricles with a pace than as a resultof an intrinsic contraction. In that sense, conventional bradycardiapacing of an atrium and/or a ventricle may be considered asresynchronization therapy. Resynchronization pacing, however, may alsoinvolve pacing in accordance with a synchronized pacing mode asdescribed below.

[0025] It is advantageous to deliver resynchronization therapy inconjunction with one or more synchronous bradycardia pacing modes, suchas are described above. Resynchronization therapy may then beimplemented by adding synchronized pacing to the bradycardia pacing modewhere paces are delivered to one or more synchronized pacing sites in adefined time relation to one or more selected sensing and pacing eventsthat either reset escape intervals or trigger paces in the bradycardiapacing mode. One heart chamber is designated as the rate chamber, andthe heart chamber contralateral to the rate chamber is designated as asynchronized chamber. For example, the right ventricle may be designatedas the rate ventricle with the left ventricle designated as thesynchronized ventricle, and the paired atria may be similarlydesignated. The synchronized chamber is then paced with an inhibiteddemand pacing mode using an escape interval that is reset by a senseoccurring in the contralateral rate chamber or a pace delivered toeither chamber.

[0026] One synchronized pacing mode may be termed offset synchronizedpacing. In this mode, the synchronized chamber is paced with a positive,negative, or zero timing offset as measured from a pace delivered to itspaired rate chamber, referred to as the synchronized chamber offsetinterval. The offset interval may be zero in order to pace both chamberssimultaneously, positive in order to pace the synchronized chamber afterthe rate chamber, or negative to pace the synchronized chamber beforethe rate chamber. One example of such pacing is biventricular offsetsynchronized pacing where both ventricles are paced with a specifiedoffset interval. The rate ventricle is paced in accordance with asynchronous bradycardia mode which may include atrial tracking, and theventricular escape interval is reset with either a pace or a sense inthe rate ventricle. (Resetting in this context refers to restarting theinterval in the case of an LRL ventricular escape interval and tostopping the interval in the case of an AVI.) Thus, a pair ofventricular paces are delivered after expiration of the AVI escapeinterval or expiration of the LRL escape interval, with ventricularpacing inhibited by a sense in the rate ventricle that restarts the LRLescape interval and stops the AVI escape interval.

[0027] As mentioned above, certain patients may experience some cardiacresynchronization from the pacing of only one ventricle and/or oneatrium with a conventional bradycardia pacing mode. It may be desirable,however, to pace a single atrium or ventricle in accordance with apacing mode based upon senses from the contralateral chamber. This mode,termed synchronized chamber-only pacing, involves pacing only thesynchronized chamber based upon senses from the rate chamber. One way toimplement synchronized chamber-only pacing is to pseudo-pace the ratechamber whenever the synchronized chamber is paced before the ratechamber is paced, such that the pseudo-pace inhibits a rate chamber paceand resets any rate chamber escape intervals. Such pseudo-pacing can becombined with the offset synchronized pacing mode using a negativeoffset to pace the synchronized chamber before the rate chamber and thuspseudo-pace the rate chamber, which inhibits the real scheduled ratechamber pace and resets the rate chamber pacing escape intervals. Sensedevents in the rate chamber will thus inhibit the synchronizedchamber-only pacing, which may benefit some patients by preventingpacing that competes with intrinsic activation (i.e., fusion beats). Inorder to prevent pacing the synchronized chamber during its vulnerableperiod, a synchronized chamber pace may be inhibited if a synchronizedchamber sense occurs within a protection period prior to expiration ofthe rate chamber escape interval. Since the synchronized chamber pace isinhibited by the protection period, the rate chamber is not pseudo-pacedand, if no intrinsic activity is sensed in the rate chamber, it will bepaced upon expiration of the rate chamber escape intervals. The ratechamber pace in this situation may thus be termed a safety pace. Forexample, in left ventricle-only synchronized pacing, a right ventricularsafety pace is delivered if the left ventricular pace is inhibited bythe left ventricular protection period and no right ventricular sensehas occurred.

[0028] An example of synchronized chamber-only pacing is leftventricle-only synchronized pacing where the rate and synchronizedchambers are the right and left ventricles, respectively. Leftventricle-only synchronized pacing may be advantageous where theconduction velocities within the ventricles are such that pacing onlythe left ventricle results in a more coordinated contraction by theventricles than with conventional right ventricular pacing orbiventricular pacing. Left ventricle-only synchronized pacing may beimplemented in inhibited demand pacing modes with or without atrialtracking. A left ventricular pace is then delivered upon expiration ofthe AVI escape interval or expiration of the LRL escape interval, withleft ventricular pacing inhibited by a right ventricular sense thatrestarts the LRL escape interval and stops the AVI escape interval.

[0029] 4. Timing Refractory Period

[0030] When a pace is delivered to the synchronized chamber, the pacewill be received by the rate chamber sensing channel as a far-fieldsense unless the channel is rendered refractory. Accordingly, across-chamber sensing refractory period for the rate chamber sensingchannel may be provided that is initiated upon delivery of a pace to thesynchronized chamber. The sensing refractory period may be made up of ablanking interval, during which the sensing amplifiers are blanked andno signals are received, and/or a software implemented refractory periodduring which received signals are simply ignored by the device. Thesensing refractory period thus prevents a far-field sense due to a pacefrom being counted for purposes of detecting a tachyarrhythmia.

[0031] In a synchronized chamber-only pacing mode, the depolarizationresulting from a pace delivered to the synchronized chamber is conductedto the rate chamber and causes depolarization there also if the ratechamber has not been earlier depolarized by intrinsic activation (e.g.,conduction of excitation from the AV node in the case of a ventricle).If the conducted depolarization is detected by the rate chamber sensingchannel, the rate chamber sense will be used to reset the pacing escapeinterval. Since a conducted depolarization is part of the same cardiaccycle as the pace delivered to the synchronized chamber, the resettingof the escape interval effectively lengthens the pacing interval andhence lowers the pacing rate below that which is desired. The pacinginterval is lengthened by the conduction time between the pacedsynchronized chamber and the rate chamber.

[0032] In the following example, the right and left ventricles are therate and synchronized chambers, respectively. The pacing interval isincreased by the LV-to-RV conduction time when the conducteddepolarization is sensed by the RV sensing channel. The following tableillustrates the effective pacing rate for conduction times of 150, 250,and 250 ms and intended pacing rates of 50, 100, and 150 ppm: LV-to-RVConduction Time Intended Rate 150 250 350 50 44 41 39 100 80 71 63 150109 92 80

[0033] This undesirable decrease in the pacing rate can be avoided bylengthening the sensing refractory period so that conducteddepolarizations fall within the period and are not sensed by the RVsensing channel. FIG. 2A illustrates this situation where an RV senseresulting from a conducted LV pace occurs within the sensing refractoryperiod SRP and thus goes unsensed with no resetting of the escapeinterval. The intended pacing interval is thus unaffected. In order toensure that ventricular tachyarrhythmias can be detected by measuringthe rate at which intrinsic depolarizations occur, however, it isnecessary that the sensing refractory period be no longer than one-halfof the escape interval. At high pacing rates, such as may occur inrate-adaptive pacing during periods of high metabolic demand, thesensing refractory period cannot be lengthened enough to avoid sensingconducted depolarizations if the LV-to-RV conduction time is long. Forexample, with a pacing rate of 150 ppm, the sensing refractory periodcan be no longer than 200 milliseconds without compromising detection ofventricular tachyarrhythmias. FIG. 2B illustrates this situation wherethe RV sense from the conducted LV pace occurs outside the sensingrefractory period and resets the escape interval. A new cardiac cycle isthen started at the time of the RV sense that effectively lengthens thepacing interval.

[0034] In accordance with the invention, a timing refractory period forthe rate chamber sensing channel initiated by a synchronized chamberpace is implemented in order to prevent resetting of the escapeinterval. During this timing refractory period, senses are ignored forpurposes of resetting the escape interval but counted for purposes ofdetecting a tachyarrhythmia. The timing refractory period can thus be aslong as necessary to avoid resetting of the escape interval by senses ofconducted depolarizations (e.g., the timing refractory period can begreater than one-half of the escape interval), while the sensingrefractory period can be made short enough that tachyarrhythmias aredetectable at all pacing rates. FIG. 3A shows the operation of thetiming refractory period when no ventricular tachyarrhythmia is present.An RV sense from a conducted LV pace is shown occurring outside of thesensing refractory period SRP but within the timing refractory periodTRP. There is therefore no lengthening of the pacing interval, and thepresence of a ventricular tachyarrhythmia can be checked for bymeasuring the interval between RV senses. FIG. 3B shows the operation ofthe timing refractory period during a ventricular tachyarrhythmia. An RVsense from a conducted LV pace again occurs outside of the sensingrefractory period SRP but within the timing refractory period TRP sothat no lengthening of the pacing interval occurs. Only the intervalsbetween successive RV senses are used for detection of a ventriculartachyarrhythmia, including the two RV senses that straddle the LV pace.When a ventricular tachyarrhythmia is detected, the device controllercan cease bradycardia pacing and/or initiate anti-tachycardia pacing.The former action may be implemented by discontinuing the timingrefractory period to enable the RV senses to inhibit further bradycardiapacing.

[0035] Although the invention has been described in conjunction with theforegoing specific embodiment, many alternatives, variations, andmodifications will be apparent to those of ordinary skill in the art.Such alternatives, variations, and modifications are intended to fallwithin the scope of the following appended claims.

1-20. (Canceled).
 21. A method for operating a cardiac rhythm managementdevice, comprising: sensing a heart chamber designated as the ratechamber and generating sense signals upon detection of depolarizationoccurring in the rate chamber; pacing a heart chamber contralateral torate chamber, designated as the synchronized chamber, with an escapeinterval that is reset by either a synchronized chamber pace or a ratechamber sense; rendering the rate chamber sensing channel refractory fora selected timing refractory period upon delivery of a synchronizedchamber pace during which rate chamber senses are ignored for purposesof resetting the escape interval but counted for purposes oftachyarrhythmia detection.
 22. The method of claim 21 further comprisingrendering the rate chamber sensing channel refractory for a selectedcross-chamber sensing refractory period upon delivery of a synchronizedchamber pace during which no sensing of rate chamber events occurs, thecross-chamber sensing refractory period being shorter than the timingrefractory period.
 23. The method of claim 21 wherein right and leftventricles are the rate and synchronized chambers, respectively.
 24. Themethod of claim 21 wherein the paired atria are the rate andsynchronized chambers.
 25. The method of claim 21 wherein thesynchronized chamber is paced in accordance with a synchronizedchamber-only pacing mode.
 26. The method of claim 21 wherein the timingrefractory period is greater than one-half of the escape interval. 27.The method of claim 21 further comprising detecting a tachyarrhythmia bymeasuring intervals between successive rate chamber senses.
 28. Themethod of claim 22 wherein the cross-chamber sensing refractory periodis at least partially implemented by blanking a sensing amplifier of therate chamber sensing channel.
 29. The method of claim 21 furthercomprising discontinuing the rate chamber timing refractory period if atachyarrhythmia in the rate chamber is detected.
 30. The method of claim29 further comprising initiating anti-tachycardia pacing or shocktherapy if a tachyarrhythmia in the rate chamber is detected.
 31. Acardiac rhythm management device, comprising: a sensing channel forsensing depolarizations from a heart chamber designated as the ratechamber; a pacing channel for delivering paces to a heart chambercontralateral to the rate chamber, designated as the synchronizedchamber; a controller programmed to pace the synchronized chamber withan escape interval that is reset by either a synchronized chamber paceor a rate chamber sense; and, wherein the controller is programmed todetect tachyarrhythmias by measuring the intervals between rate chambersenses and to render the rate chamber sensing channel refractory for aselected timing refractory period upon delivery of a synchronizedchamber pace during which rate chamber senses are ignored for purposesof resetting the escape interval but counted for purposes oftachyarrhythmia detection.
 32. The device of claim 31 wherein thecontroller is programmed such that the rate chamber sensing channel isrendered refractory for a selected cross-chamber sensing refractoryperiod upon delivery of a synchronized chamber pace during which nosensing of rate chamber events occurs, the cross-chamber sensingrefractory period being shorter than the timing refractory period. 33.The device of claim 31 wherein the controller is programmed such thatthe right and left ventricles are the rate and synchronized chambers,respectively.
 34. The device of claim 31 wherein the controller isprogrammed such that the paired atria are the rate and synchronizedchambers.
 35. The device of claim 31 wherein the controller isprogrammed such that the synchronized chamber is paced in accordancewith a synchronized chamber-only pacing mode.
 36. The device of claim 31wherein the controller is programmed such that the timing refractoryperiod is greater than one-half of the escape interval.
 37. The deviceof claim 31 wherein the controller is programmed such that atachyarrhythmia is detected by measuring intervals between successiverate chamber senses.
 38. The device of claim 32 wherein thecross-chamber sensing refractory period is at least partiallyimplemented by blanking a sensing amplifier of the rate chamber sensingchannel.
 39. The device of claim 31 wherein the controller is programmedsuch that the rate chamber timing refractory period is discontinued if atachyarrhythmia in the rate chamber is detected.
 40. The device of claim31 wherein the controller is programmed such that anti-tachycardiapacing or shock therapy is initiated if a tachyarrhythmia in the ratechamber is detected.